This invention relates to the nondestructive evaluation of geometrical characteristics of machined parts. More particularly, this invention relates to apparatus for the dimensional inspection of apertures or holes within production parts to determine whether such apertures comply with various geometric and dimensional design constraints or tolerances.
In many situations, it is necessary to precisely bore or otherwise machine apertures or holes into or through a production workpiece or part such that the various geometric and dimensional characteristics of the aperture do not deviate from nominal or ideal values by more than a predetermined amount. Such geometrical and dimensional characteristics include, for example, the size of an aperture (e.g., diameter at the surface of the workpiece or any other reference plane), contour of the aperture walls (e.g., axial straightness and taper or other various profile characteristics), cross-sectional geometry of the aperture in one or more reference planes (e.g., roundness) and orientation of the axial centerline of the aperture relative to the workpiece or other reference (e.g., perpendicularity). Since the acceptability and reliability of a finished product that incorporates a component containing precisely formed apertures can be affected if the apertures do not conform to design objectives, it is often necessary to perform a detailed inspection of such apertures prior to utilizing a component part in which they are formed. Where a number of geometrical and dimensional characteristics are important and/or a large number of apertures must be evaluated, such inspection is not only a time-consuming and expensive task, but, when performed by conventional manual measurement techniques, is subject to error.
One situation which necessitates apertures or holes that exhibit very precise dimensional and geometrical characteristics relates to tapered fastener systems of the type utilized to join structural components in aircraft and other stuctures that are subject to substantial operational stress and strain. In this regard, conventional tapered fasteners are precisely dimensioned and require apertures of corresponding preciseness so that, when the fastener is drawn into an aperture, a controlled amount of interference occurs between the fastener and the wall of the aperture. This interference produces compressive stress which joins the components together in a manner that provides high resistance to structural fatigue and thereby extends the service life of the assembled parts over that which would result through the use of other conventional fastening techniques.
To ensure that the advantages of such a tapered fastener system are achieved, it is generally necessary to evaulate several geometrical and dimensional characteristics of apertures that are to receive tapered fasteners. In this respect, geometrical characteristics such as the eccentricity or roundness of the aperture, the taper and straightness of axial elements within the interior walls of the aperture and the orientation of the aperture axial centerline relative to the surface of the part must be determined as well as the diameter of the aperture relative to various cross-sectional datum planes.
Although various prior art measurement apparatus and techniques have been utilized in the inspection or evaluation of apertures for tapered fasteners, these apparatus and techniques have not provided a quantitative evaluation of all the important aperture characteristics and, further, serious limitations have been encountered. For example, in one inspection technique known as a "blue pin bearing check", a precisely dimensioned tapered pin of a size that corresponds to the type of tapered fastener opening being inspected is coated with a slow-drying blue dye material and the pin is inserted into the aperture to be inspected and pushed or driven inwardly so that it positively contacts the wall of the aperture. The pin is then removed and rolled on a piece of white paper to remove any dye that remains on the pin. The amount of dye that is transferred to the taper is evaluated by the person performing the inspection, e.g., by comparing this particular dye sample with a set of reference samples, to thereby provide an indication of the amount of bearing area existing between the tapered pin and the aperture being inspected and hence the percent of surface area where interference will occur when a tapered fastener is installed in the aperture. Additionally, a technique known as "head protrusion gauging" has been utilized wherein specially dimensioned protrusion gauges measure the protrusion of the head above the surface of the workpiece or production part where the fastener is inserted into an aperture just prior to installation. Such a technique is similar to the blue pin bearing technique except that it provides a qualitative indication of the amount of interference between the bolt and the aperture being inspected. Further, these techniques and others utilized heretofore not only require a substantial amount of subjective evaluation and experience on the part of the individual performing the inspection, but require a significant amount of time. For these and other reasons, prior art inspection apparatus and techniques for evaluating precisely machined apertures such as those for receiving tapered fasteners have not been totally satisfactory. This is especially true in production situations wherein a large number of such apertures must be machined and inspected within a reasonable amount of time and, insofar as possible, without requiring special skills and training not possessed by typical production and inspection personnel.
Accordingly, it is an object of this invention to provide an automatic inspection system for determining various geometrical and dimensional characteristics of apertures or bores contained in workpieces or production parts.
It is another object of this invention to provide an automatic inspection system of the above described type which can be utilized for rapid evaluation of apertures in a production environment.
It is yet another object of this invention to provide an inspection system for the evaluation of apertures in production parts wherein geometrical and dimensional data is automatically gathered and evaulated within digital computation apparatus.